If we take three equal resistors and connect one end of each to a common point, then apply three input voltages, the voltage seen at the common point will be the mathematical average of the three.
This circuit is commonly known as a PASSIVE AVERAGER, because it generates an average voltage with non-amplifying components. Passive simply means that it is an unamplified circuit. The large equation of the averager circuit describes the voltage produced by multiple voltage sources connected together through individual resistances. Since the three resistors in the averager circuit are equal to each other, we can simplify the equation by writing R1, R2, and R3 simply as R.
If we take a passive averager and use it to connect three input voltages into an op-amp amplifier circuit with a gain of 3, we can turn this averaging function into an addition function. The result is called a noninverting summer circuit.
With a voltage divider composed of a 2 kΩ / 1 kΩ combination, the noninverting amplifier circuit will have a voltage gain of 3. By taking the voltage from the passive averager, which is the sum of V1, V2, and V3 divided by 3, and multiplying that average by 3, we arrive at an output voltage equal to the sum of V1, V2, and V3. The reversal in polarity is what makes this circuit an inverting summer.
Summer (adder) circuits are quite useful in analog computer design, just as multiplier and divider circuits would be. Again, it is the extremely high differential gain of the op-amp which allows us to build these useful circuits with a bare minimum of components.
No comments:
Post a Comment